Dropwise condensation



Filed March 51, 1965 STEAM y INVENTORS EDMUND THELE/V EDGAR A. CADWALLADER ATTORNEYS United States Patent 3,305,007 DROPWISE CUNDENSATION Edmund Thelen, Philadelphia, Pa., and Edgar A. Cadwallader, Silver Spring, Md., assiguors to the United States of America as represented by the Secretary of the Interior Filed Mar. 31, 1965, Ser. No. 444,950 4 Claims. (Cl. 165133) The invention herein described and claimed may be manufactured and used by or for the Government of the United Staes of America for governmental purposes without the payment of royalties thereon or therefor.

This invention is concerned with promotion of dropwise formation of water on the surfaces of heat transfer devices, particularly condensers.

Advantages of dropwise condensation over film-type condensation, particularly in decreased thermal resistance between vapors to be condensed and the cooling fluid, is well known in the prior art, e.g., US. Patent No. 2,248,909. This patent discloses the use of zirconium and tantalum to promote dropwise condensation. Other materials have also been employed for this purpose, e.g., chromium (U.S.'Patent 2,259,024), oleic acid (U.S. Patent 2,919,115), resins such as polyfluoroethylene (US. Patent 2,923,640) and copper sulfide (Trans. A.I.Ch.E., vol. 31, No. 4, December 1935, pages 59362l).

It has now been found that very effective dropwise condensation on condenser surfaces may be achieved by the use of silver sulfide (sulfided silver) as the surface on which the water condenses. This surface may be achieved by sulfiding solid silver or by coating another metal with silver followed by sulfiding.

Silver coatings are usually achieved by means of electroplating. A base coat of nickel, copper-nickel alloy, or steel is usually advisable to enable the use of smaller amounts of silver. Any application technique from which a final smooth surface can be obtained may be employed. This could include, in addition to electrode position, vacuum deposition (including sputtering), chemical deposition, mechanical cladding or flame spraying.

Sulfiding may be accomplished by any conventional means. Treatment with hydrogen sulfide gas in moist air, as was employed in the example below, has been found to product uniform, adherent sulfide films. Concentrations of H S from 1 percent to 70 percent by volume may be used. Other methods of forming sulfide films include reaction with aqueous Na s and with sulfur dissolved in an organic disulfide. Thickness of the sulfide film will usually vary from about .05 to 0.5 microns.

Although distilled water was used in the examples below, the method of the invention is also beneficial in promoting dropwise condensation of vapor distilled from water containing impurities, e.g., sea water.

An accompanying drawing, which is not to scale, schematically illustrates a device according to the present invention. In that drawing, represents a metal substrate 3,305,007 Patented Feb. 21, 1967 which may be steel or other metal as explained above. 12 represents a silver sulfide coating. Steam is shown dropwise condensing on the silver sulfide surface when the substrate is cooled with a cooling fluid such as water.

The invention will be more specifically illustrated by means of the following examples.

Example 1 The heat transfer apparatus used in this example consisted of a copper block having cooling water flowing through a hollowed out central portion. The block was suspended over boiling water in a heated vessel under atmospheric pressure. Attached to the block were 42 test 1x 3 inch metal flats consisting of various test materials on different substrates, having a total thickness of about 0.05 inch. The block and flats were arranged so that the flats were disposed vertically over the boiling Water. Steam arising from the boiling water condensed on the metal flats, which were in thermal transfer relationship with the cooled copper block. The water which condensed on the test flats dripped back into the boiling water.

Samples tested and results are given in Table l. The silver flats (prior to sulfiding) were commercially rolled and polished sheet stock cleaned with detergent and rinsed with pure ethanol. The electroplated coatings were approximately 0.001 inch thick and were polished with Nos. 400 and 600 aluminum oxide and jewelers rouge. The dropwise quality, Q, defined by the following equation, has been found to give a reliable measure of the ability of a surface to promote dropwise condensation.

Q=l.0 times the precent sample area which is dropwise condensing +0.5 times the precent mixed area.

Drop appearance in the table is based on the following scale:

E (Excellent)Drops have circular base line and high contact angle. Sliding drops are of small diameter.

G (Good)Drops nearly circular ellipses. Base line convex.

F (Fair)Drops often have base lines with straight line segments. Drops grow to large size before sliding.

P (Poor)Drop base lines often have concave segments.

Usually tracks and/0r droplet residues result in sweeping. This class when seen is often a precursor to mixed condensation.

As is apparent from the data of Table 1, the silver sulfide surfaces exhibit dropwise condensation even after long periods of exposure to condensation and are much superior to copper sulfide surfaces.

The invention may be used in any heat transfer device in which heat is removed from a vapor through a heat transferring wall by means of a suitable cooling fluid, either liquid or gaseous, the vapor being condensed on the heat transferring wall. Examples of such devices are heat exchangers, evaporators, condensers, tubular heaters, tempering coils, etc.

TABLE 1.CONDENSATION BEHAVIOR AS A FUNCTION OF TIME 1 Day 14 Days 22 Days 28 Days Sample Type Q, D rop Q, Drop Q Drop Q, D rop Appearance Appearance Appearance Appearance Ag coated on steel, light sulfide 97. G- 97. 5 75 F+ Cu coated on steel, light sulfide 95 F+ 47. 5 Silver (polished), medium sulfide 97. 5 G t. 97. 5 95 F- Copper (polished), medium sulfide 2 92. 5 G 45 35 Days 47 Davs 69 Days Days 283 Days Sample Type Q Drop D rop Q D rop Q Drop Q Drop Appearance Appearance Appearance Appearance Appearance Ag coated on steel, light sulfide 1 72. 5 Ag coated on steel, medium sulfide t G. On coated on steel, light sulfide 2. 5 Silver (polished), medium sulfide Copper (polished), medium sulfide 35 1 Thickness of sulfide film was about 0.08 microns. 2 Thickness of sulfide film was about 0.15 microns.

What is claimed is:

1. In a heat-transfer device wherein water vapor is condensed by contacting a cooling surface, the improvement comprising employing silver sulfide as the cooling surface.

2. In a heat transfer device comprising a cooling fluid zone, a water vapor zone having a temperature greater than that of said cooling fluid zone, and a heat conducting impermeable partition disposed between said zones, whereby mass transfer between said zones is prevented, said partition having a first surface in contact with said cooling fluid zone and a second surface in contact with said water vapor zone, the improvement comprising employing silver sulfide as said second surface.

References Cited by the Examiner UNITED STATES PATENTS 7/1941 Russell -133 9/ 1965 Ncugebauer et a1. 165-133 35 ROBERT A. OLEARY, Primary Examiner.

C. SUKALO, Assistant Examiner. 

1. IN A HEAT-TRANSFER DEVICE WHEREIN WATER VAPOR IS CONDENSED BY CONTACTING A COOLING SURFACE, THE IMPROVEMENT COMPRISING EMPLOYING SILVER SULFIDE AS THE COOLING SURFACE. 